1. Field of the Invention
This invention relates to top nozzles for nuclear fuel assemblies which accommodate for differences in thermal expansion and irradiation growth of fuel assemblies and other reactor components, and in particular, to a retrofit expandable top nozzle for use in reactors previously having components composed of essentially the same materials. The invention also relates to a device for securing an expandable top nozzle to a fuel assembly.
2. Background of Information
In nuclear reactors of the type designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies each having a plurality of fuel rods held in an organized hexagonal array by a plurality of grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimbles. The stainless control rod guide thimbles extend above and below the ends of the fuel rods and are attached to top and bottom nozzles, respectively. The fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles.
The top nozzles in the Soviet design are non-removably fixed to the stainless steel control rod guide thimbles of the fuel assembly. These complex nozzles perform several major functions. First, they position the remote control cluster assembly (RCCA) relative to the guide tubes within the core so that the position of the RCCA relative to the upper core plate is fixed. The RCCA positions the control rods which are inserted into the fuel assembly as a group or cluster.
The Soviet nozzle also dampens the velocity of the control rods using a spring to remove energy when the rods are dropped into the reactor core during an emergency shutdown. The nozzle also supplies spring loads for supporting the internals. When the upper core plate is lowered onto the nozzles, it compresses the nozzle spring. In addition, the Soviet nozzle is designed to protect the control rods when the fuel assembly is removed from the reactor vessel. Under these conditions, the RCCA is at or below the top edge of the nozzle. Finally, the Soviet design of the top nozzle allows the fuel assembly to be handled when lifted out of the core by transferring the loads through the nozzle.
Thus, the Soviet nozzle is designed to function in two positions; free and compressed. As stainless steel is used for the thimbles of the Soviet fuel assembly, the relative separation between the interior of the reactor vessel and the fuel assemblies remains constant once the assembly is in position. Spring loads are such that the nozzles can support the internals, and the spring loads as well as the RCCA positions are fixed so that all functions are static. As a result, the nozzle has built-in references around which the internals are designed.
The stainless steel thimbles used in the Soviet design impose higher reactivity cost on the fuel assemblies, and they are more difficult to attach to the grids of the fuel assemblies. Non-Soviet fuel assemblies utilize zircaloy for the thimbles which imposes less reactivity cost. However, zircaloy has a different constant of thermal expansion than the stainless steel reactor vessel, and grows during irradiation. Expandable top nozzles which accommodate for these variations in the dimensions of different components within the reactor, are disclosed in, for example, U.S. Pat. Nos. 4,534,933; 4,687,619; 4,702,882 and 4,986,959. Such nozzles, however, are used in reactors in which the top core plate rests on a core support in the form of a circumferential ledge within the reactor vessel. In the Soviet-type, reactor, the core plate rests on and is supported by the top nozzles.
As mentioned, the Soviet design top nozzle is permanently attached to the thimble tubes of the fuel assembly. The above-mentioned patents disclose removable top nozzles. U.S. Pat. No. 4,641,409 discloses a method for attaching a replacement nozzle in which the thimble sleeve is circumferentially bulged into an annular groove in the adapter plate of the replacement nozzle. In another design, insert sleeves are secured to the ends of the guide thimbles. Each insert has an annular outwardly extending bulge or rib adjacent its free end and is slotted from its free end to form flexible fingers. These flexible fingers deflect when the insert is inserted into the apertures in the adapter plate and then expand outward to engage the annular rib or bulge in the annular groove in the adapter plate bore. Lock tubes are inserted inside the insert to maintain engagement of the annular rib or bulge and the groove of the adapter plate.
In order to remove the nozzles, such as to replace damaged fuel rods, the lock tube is removed and the adapter plate and fuel assembly are pulled apart. Removal of the top nozzle with a slotted insert has been difficult because the forces required to disjoint the nozzle from the fuel assembly have occasionally been excessive. As such replacement of the fuel rods is typically performed dining a critical path refueling outage, any difficulty in removing the nozzle results in higher cost to the reactor operator. In addition, once the top nozzle is removed, the protruding inserts are left exposed to rod replacement tooling, subjecting the inserts to possible damage, although actual frequency of damage from replacement tooling is low. Also, each of the lock tubes must be removed individually and each tube requires a separate inspection prior to replacement.
There is a need for an improved removable top nozzle and the mechanism for securing the same to a nuclear fuel assembly.
More particularly, there is a need for an improved top nozzle of the Soviet design which is removable.
There is a further need for such an improved top nozzle which can be used with a fuel assembly having zircaloy guide thimbles.
There is a further need for such an improved top nozzle which otherwise maintains all the functions of the Soviet-type top nozzle.
There is another need for such a top nozzle which has reduced hydraulic resistance.
There is an additional need for an improved device for removably securing the top nozzle to the fuel assembly guide thimbles.
More particularly, there is a need for such an improved locking device which has low insertion and withdraw forces, yet firmly locks the nozzle to the fuel assembly.